Method for controlling a semiconductor device having CAL latency function

ABSTRACT

One semiconductor device includes a command receiver receiving the command signal to generate a first internal command signal, and a latency control circuit activating a second internal chip select signal after elapse of first cycles of a clock signal since a first internal chip select signal is activated. The latency control circuit activates a second control signal when the chip select signal is maintained in an inactive state during second cycles of the clock signal that is larger than the first cycles. The command receiver is activated based on a first control signal. The first control signal is activated in response to the first internal chip select signal. The first control signal is deactivated in response to the second control signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a Continuation Application of U.S. patentapplication Ser. No. 14/733,924 filed on Jun. 8, 2015, which is aContinuation Application of U.S. patent application Ser. No. 14/268,827filed on May 2, 2014, now U.S. Pat. No. 9,053,775 issued on Jun. 9,2015, which is a Continuation Application of U.S. patent applicationSer. No. 13/615,430 filed on Sep. 13, 2012, now U.S. Pat. No. 8,743,652issued on Jun. 3, 2014, which is based on and claims priority fromJapanese Patent Application No. 2011-212142, filed on Sep. 28, 2011, allof which are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a semiconductor device and aninformation processing system including the same, and more particularlyto a semiconductor device that can issue a command signal and the likeat timing different from that of a chip select signal and an informationprocessing system including the same.

Description of Related Art

Semiconductor memory devices typified by a dynamic random access memory(DRAM) receive an address signal and a command signal supplied from acontroller, and access the memory cell array based on the signals. Theaddress signal and the command signal are enabled when a chip selectsignal supplied from the controller is activated. In principle, thecontroller therefore needs to issue the address signal and the commandsignal with the chip select signal activated.

DDR4 (Double Data Rate 4) DRAMs have recently been proposed as DRAMseven faster than DDR3 (Double Data Rate 3) DRAMs DDR4 DRAMs support anew function called “CS_n to command address latency (CAL latency)”. TheCAL latency means that the controller supplies an address signal and acommand signal to a semiconductor device after a predetermined time(predetermined latency) since the controller supplies a chip selectsignal to the semiconductor device and the semiconductor device receivesthe address signal and the command signal after a predetermined time(predetermined latency) since the reception of the chip select signal.That is, the CAL latency is a function that allows input of the addresssignal and the command signal after a lapse of a predetermined latencysince the reception of the chip select signal with respect to thesemiconductor device. Such a function can be used to grasp the inputtiming of the address signal and the command signal on the semiconductordevice (semiconductor memory device) side. Address receivers and commandreceivers can thus be deactivated in periods where the address signaland the command signal are not input. This allows a reduction in powerconsumption.

A semiconductor device that can issue a command signal and the like attiming different from that of a chip select signal is also described inJapanese Patent Application Laid-Open No. 2000-285674.

What timing to change the address receivers and the command receiversfrom an inactive state to an active state after a lapse of the CALlatency since the activation of the chip select signal, and what timingto change the receivers from an active state to an inactive state, maybe determined in consideration of the CAL latency and power consumption.There are three important factors concerned, including the powerconsumption of the receivers, the power consumption caused by controlsignals for controlling the activation and deactivation of thereceivers, and the value of the CAL latency.

For example, Japanese Patent Application Laid-Open No. 2000-285674discusses that an enable signal is activated at timing ½ clock cyclesafter the activation of a chip select signal, and the enable signal isdeactivated at timing one clock cycle later according to thesemiconductor device described in Japanese Patent Application Laid-OpenNo. 2000-285674, the enable signal has a waveform that changes with thechip select signal. If the chip select signal changes frequently in ashort period, the enable signal also changes frequently in a short time.In such a case, the semiconductor device fails to provide a sufficienteffect for reducing power consumption including the charging anddischarging currents of the enable signal. The reason is that the enablesignal, supplied to a large number of address receivers and commandreceivers, is a high-load internal signal in the semiconductor device.

SUMMARY

In one embodiment, there is provided a semiconductor device having amemory array, the method includes receiving a mode register command toset a command latency value in a mode register; receiving a chip selectsignal; activating a command receiver in response to the chip selectsignal; receiving, with the command receiver, an access command with afirst latency from the chip select signal equal to the command latencyvalue; accessing the memory array in response to the access command; anddeactivating the command receiver with a second latency from the chipselect signal equal to a deactivation latency value.

In another embodiment, there is provided a semiconductor device thatincludes: a command receiver receiving a command signal supplied fromoutside via a command terminal to generate an internal command signal;and a control circuit activating an internal chip select signal afterelapse of a first latency since a chip select signal supplied fromoutside via a chip select terminal is activated. The control circuitactivates the command receiver in response to the chip select signal,and deactivates the command receiver when the chip select signal ismaintained in an inactive state during a second latency that is largerthan the first latency.

In still another embodiment, there is provided an information processingsystem that includes: a controller that outputs a chip select signal anda command signal; and a semiconductor device. The semiconductor deviceincludes: a command receiver receiving the command signal supplied fromthe controller to generate an internal command signal; and a controlcircuit activating an internal chip select signal after elapse of afirst latency since the chip select signal supplied from the controlleris activated. The control circuit activates the command receiver inresponse to the chip select signal, and deactivates the command receiverwhen the chip select signal is maintained in an inactive state during asecond latency that is larger than the first latency.

According to the present invention, the active state of the commandreceiver is maintained even if the chip select signal changes frequentlyin a short period. This reduces charging and discharging currentsresulting from frequent repetitions of activation and deactivation ofthe command receiver, whereby the power consumption of the semiconductordevice can be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram for explaining the principle of an embodimentof the present invention;

FIG. 2 is a block diagram indicative of an embodiment of a semiconductordevice 10 a according to a first preferred embodiment of the presentinvention and mainly shows details of circuit blocks belonging to anaccess control circuit 20 shown in FIG. 1;

FIG. 3 is a circuit diagram indicative of an embodiment of a latencycontrol circuit 100;

FIG. 4 is a circuit diagram indicative of an embodiment of a receivercontrol circuit 200;

FIG. 5 is an example of a truth table for explaining an operation of acommand decoder 82;

FIG. 6 is a timing chart for explaining the operation of thesemiconductor device 10 a according to the first embodiment and showsoperations in a CALON mode;

FIG. 7 is a block diagram indicative of an embodiment of a semiconductordevice 10 b according to a second preferred embodiment of the presentinvention and mainly shows details of circuit blocks belonging to theaccess control circuit 20 shown in FIG. 1; and

FIG. 8 is a timing chart for explaining the operation of thesemiconductor device 10 b according to the second embodiment and showsoperations in a CALON mode.

DETAILED DESCRIPTION OF THE EMBODIMENTS

A representative example of the technical concept of an embodiment ofthe present invention for solving the problem will be described below.It will be understood that what the present invention claims are notlimited to such a technical concept but set forth in the claims of thepresent invention. More specifically, the present embodiment includes:changing a command receiver from an inactive state to an active state inresponse to the activation of a chip select signal; and changing thecommand receiver from the active state to the inactive state on thecondition that the chip select signal has been maintained in an inactivestate for a time (second latency) longer than a CAL latency (firstlatency). In other words, the active state of the and receivercorresponding to a first chip select signal supplied to thesemiconductor device is changed to the inactive state on the conditionthat a second chip select signal is not supplied (i.e., the chip selectsignal has been maintained in an inactive state) in a second latencyperiod corresponding to the first chip select signal. Consequently, theactive state of the command receiver is maintained even if the chipselect signal changes frequently in a short period. This reducescharging and discharging currents resulting from the control of thecommand receiver.

Referring now to FIG. 1, an information processing system including acontroller 50 and a semiconductor device 10 is shown. The semiconductordevice 10 shown in FIG. 1 is a semiconductor memory device such as aclock synchronization type DRAM. The semiconductor device 10 includes amemory cell array 11. The memory cell array 11 includes a plurality ofword lines WL and a plurality of bit lines BL which intersect eachother. Memory cells MC are arranged at the intersections. The word linesWL are selected by a row decoder 12. The bit lines BL are selected by acolumn decoder 13. The bit lines BL are connected to respectivecorresponding sense amplifiers SA in a sense circuit 14. Bit lines BLselected by the column decoder 13 are connected to an amplifier circuit15 through sense amplifiers SA.

The operation of the row decoder 12, the column decoder 13, the sensecircuit 14, and the amplifier circuit 15 is controlled by an accesscontrol circuit 20. An address signal ADD, a command signal CMD, a chipselect signal CS, and a clock signal CK are supplied to the accesscontrol circuit 20 through terminals 21 to 24. Based on such signals,the access control circuit 20 controls the row decoder 12, the columndecoder 13, the sense circuit 14, the amplifier circuit 15, and a datainput/output circuit 30.

Specifically, if the command signal CMD is an active command, theaddress signal ADD is supplied to the row decoder 12. In response tothis, the row decoder 12 selects a word line WL that is designated bythe address signal ADD, whereby corresponding memory cells MC areconnected to respective bit lines BL. The access control circuit 20 thenactivates the sense circuit 14 at predetermined timing.

On the other hand, if the command signal CMD is a read command or awrite command, the address signal ADD is supplied to the column decoder13. In response to this, the column decoder 13 connects bit lines BLdesignated by the address signal ADD to the amplifier circuit 15.Consequently, in a read operation, read data DQ read from the memorycell array through sense amplifiers SA is output from a data terminal 31to outside through the amplifier circuit 15 and the data input/outputcircuit 30. In a write operation, write data DQ supplied from outsidethrough the data terminal 31 and the data input/output circuit 30 iswritten to memory cells MC through the amplifier circuit 15 and senseamplifiers SA.

As shown in FIG. 1, the access control circuit 20 includes addressreceivers 90 a, command receivers 90 b, a latency control circuit, andreceiver control circuit 200.

The address receivers 90 a are circuits that accept the address signalADD supplied from the controller 50 through address terminals 21. Theaddress signal ADD includes a plurality of bits. There are a pluralityof address terminals 21 and a plurality of address receivers 90 acorresponding to the respective plurality of bits. The command receivers90 b are circuits that accept the command signal CMD supplied from thecontroller 50 through command terminals 22. The command signal CMDincludes a plurality of bits. There are a plurality of command terminals22 and a plurality of command receivers 90 b corresponding to therespective plurality of bits. The address receivers 90 a and the commandreceivers 90 b are activated based on an enable signal REN. If theenable signal REN is in an inactive state, the address receivers 90 aand the command receivers 90 b are also deactivated. Thus, powerconsumption is reduced. In the present embodiment, the enable signal RENmay be referred to as a “first control signal”.

The latency control circuit 100 is a circuit that receives the chipselect signal CS supplied from the controller 50 through the chip selectterminal 23 and delays the chip select signal CS by a predeterminedlatency. The latency control circuit 100 performs delay operations insynchronization with the clock signal CK. The clock signal CK is asynchronization signal having a predetermined period. The clock signalCK is supplied from the controller 50 through the clock terminal 24. Thelatency control circuit 100 performs two types of delay operations Afirst operation includes enabling internal address signals and internalcommand signals output from the address receivers 90 a and the commandreceivers 90 b in response to a lapse of a first latency since theactivation of the chip select signal CS. A second operation includesresetting the receiver control circuit 200 in response to the fact thatthe chip select signal CS has maintained the inactive state for a secondlatency longer than the first latency after the activation of the chipselect signal CS. In the present embodiment, the first latency may bereferred to as a “first time”.

The receiver control circuit 200 is set in response to the activation ofthe chip select signal CS. The receiver control circuit 200, when set,activates the enable signal REN. The receiver control circuit 200 isreset in response to a lapse of the foregoing second latency since theactivation of the chip select signal CS. The resetting of the receivercontrol circuit 200 deactivates the enable signal REN.

The foregoing circuit blocks operate with respective predeterminedinternal voltages as their power supply. The internal power supplies aregenerated by a power supply circuit 40 shown in FIG. 1. The power supplycircuit 40 receives an external potential VDD and a ground potential VSSsupplied through power supply terminals 41 and 42, respectively. Basedon the potentials, the power supply circuit 40 generates internalvoltages VPP, VPERI, VARY, etc. The internal potential VPP is generatedby boosting the external potential VDD. The internal potentials VPERIand VARY are generated by stepping down the external potential VDD.

The internal voltage VPP is a voltage that is mainly used in the rowdecoder 12. The row decoder 12 drives a word line WL that is selectedbased on the address signal ADD to the VPP level, thereby making thecell transistors included in memory cells MC conducting. The internalvoltage VARY is a voltage that is mainly used in the sense circuit 14.The sense circuit 14, when activated, drives either one of each pair ofbit lines to the VARY level and the other to the VSS level, therebyamplifying read data that is read out. The internal voltage VPERI isused as the operating voltage of most of the peripheral circuits such asthe access control circuit 20. The use of the internal voltage VPERIlower than the external voltage VDD as the operating voltage of theperipheral circuits reduces the power consumption of the semiconductordevice 10.

Now, the controller 50 includes an output circuit 60 and a dataprocessing circuit 70. The output circuit 60 is a circuit for supplyingthe address signal ADD, the command signal CMD, the chip select signalCS, and the clock signal CK the semiconductor device 10 throughterminals 61 to 64. The data processing circuit 70 is a circuit thatprocesses read data DQ and write data DQ input/output through a dataterminal 71. When the controller 50 accesses the semiconductor device10, the controller 50 supplies the address signal ADD and the commandCMD to the semiconductor device 10 after a lapse of a predeterminedlatency.

Consequently, in a period where neither of the address signal ADD andthe command signal CMD is supplied from the controller 50, the addressreceivers 90 a and the command receivers 90 b of the semiconductordevice 10 are deactivated. This reduces the power consumption of thereceivers 90 a and 90 b. The address receivers 90 a and the commandreceivers 90 b are not deactivated immediately after a lapse of thefirst latency since the activation of the chip select signal CS.Instead, the address receivers 90 a and the command receivers 90 b aredeactivated after a lapse of the second latency longer than the firstlatency. The receivers 90 a and 90 b are thus maintained in an activestate if the activation and deactivation of the chip select signal CSare frequently repeated in a short period. Consequently, even if theactivation and deactivation of the chip select signal CS are frequentlyrepeated in a short period, it is possible to reduce the charging anddischarging currents of the high-load enable signal REN due to thecontrol of the receivers 90 a and 90 b.

Preferred embodiments of the present invention will be explained belowin detail with reference to the accompanying drawings.

Turning to FIG. 2, the access control circuit 20 includes a chip selectreceiver 91 and a clock receiver 92 aside from the address receivers 90a and the command receivers 90 b. The chip select receiver 91 receivesthe chip select signal CS supplied from the controller 50 and generatesan internal chip select signal ICS1. The clock receiver 92 receives theclock signal CK supplied from the controller 50 and generates aninternal clock signal ICLK. The internal chip select signal ICS1 and theinternal clock signal ICLK are supplied to the latency control circuit100.

Turning to FIG. 3, the latency control circuit 100 includes a shiftregister 110 which includes three cascaded stages of flip-flop circuitsFF1 to FF3. The internal chip select signal ICS1 is supplied to theflip-flop circuit FF1, in the first stage. An internal chip selectsignal ICS2 is output from the flip-flop circuit FF3 in the final state.Since the flip-flop circuits FF1, to FF3 operate in synchronization withthe internal clock signal ICLK, the shift register 110 outputs theinternal chip select signal ICS2 after three clock cycles since theactivation of the internal chip select signal ICS1. The number of stagesof the shift register 110 corresponds to the foregoing first latency.The internal chip select signal ICS2 is supplied to one of the inputnodes of a selector 141 shown in FIG. 2.

While FIG. 3 shows the case where the first latency is three clockcycles, the first latency need not be fixed. It is preferable that thevalue of the first latency is variable by mode setting. The mode settingis performed by setting a predetermined mode signal into a mode register25 shown in FIG. 2. The mode register 25 contains set values includingone as to whether to enable or disable a CAL latency operation. If anoperation mode for enabling a CAL latency operation (CALON mode s set, amode signal CALEN is activated to a high level, for example. If anoperation mode for disabling a CAL latency operation (CALOFF mode) isset, the mode signal CALEN is deactivated to a low level, for example.

The latency control circuit 100 further includes a bit counter 120. Thebit counter 120 is a circuit that performs a count-down operation insynchronization with the internal clock signal ICLK. The resulting countvalue COUNT is supplied to a detection circuit G0 which is an OR gatecircuit. The bit counter 120 includes a set node “set” which is intendedto set the count value COUNT to an initial value. When the set node“set” is activated to a high level, the count value COUNT is preset to amaximum value. As shown in FIG. 3, an output signal CALm2 of theflip-flop circuit FF1 is supplied to the set node “set” of the bitcounter 120. This means that the bit counter 120 is preset to themaximum value after one clock cycle since the activation of the internalchip select signal ICS1.

The detection circuit G0 is a circuit that detects that the count valueCOUNT of the bit counter 120 reaches a minimum value. In the presentexample, the count value COUNT is a three-bit binary signal. Countingdown the count value COUNT seven times from a maximum value of “111(=7)” reaches a minimum value of “000 (=0)”. In other words, an outputsignal RSTa of the detection circuit G0 is activated to a low levelafter a lapse of a total of eight clock cycles since the activation ofthe internal chip select signal ICS1. Note that if the internal chipselect signal ICS1 is activated again within eight clock cycles sincethe previous activation of the internal chip select signal ICS1, thecount value COUNT returns to the maximum value. The output signal RSTais thus activated to a low level on the condition that the internal chipselect signal ICS1 has not been activated for eight consecutive clockcycles since the last activation of the internal chip select signalICS1. The number of clocks corresponds to the foregoing second latency.In the present embodiment, the second latency (the time between when thechip select signal CS is deactivated and when the enable signal REN isdeactivated to a low level) may be referred to as a “second time”.

While FIG. 3 shows the case where the second latency is eight clockcycles, the present invention is not limited thereto. Note that thesecond latency at least needs to be longer than the first latency. Thereason is that if the second latency is shorter than the first latency,the receivers 90 a and 90 b can be deactivated at timing when theaddress signal ADD or the command signal CMD is input.

The output signal RSTa of the detection circuit G0 is supplied to apulse generation circuit 130. The pulse generation circuit 130 is acircuit that activates a reset signal RST, a one-shot pulse, in responseto a change of the output signal RSTa, of the detection circuit G0 froma high level to a low level. In the present embodiment, the reset signalRST may be referred to as a “second control signal”. The second controlsignal is supplied to the receiver control circuit 200 shown FIG. 2.

Turning to FIG. 4, the receiver control circuit 200 includes an SR latchcircuit L. A NOR gate circuit G1 receives the inverted signal of themode signal CALEN and the internal chip select signal ICS1. The outputof the NOR gate circuit G1 is supplied to a set node S of the SR latchcircuit L. The inverted signal of the reset signal RST is supplied to areset node R of the SR latch circuit L. With such a configuration, ifthe mode signal CALEN is activated to a high level, i.e., set to theCALON mode and the internal chip select signal ICS1 is activated, thenthe enable signal REN is immediately activated to a high level. In thepresent embodiment, the time between when the chip select signal CS isactivated and when the enable signal REN is activated to a high levelmay be referred to as a “third time”. Subsequently, when the resetsignal RST is activated, the enable signal REN is deactivated to a lowlevel. The activation timing of the reset signal RST is as has beendescribed with reference to FIG. 3. On the other hand, if the modesignal CALEN is deactivated to a low level, i.e., set to the CALOFFmode, the enable signal REN is constantly activated to a high level.

The enable signal REN is supplied to a receiver 90 shown in FIG. 2. Thereceiver 90 is a circuit block including the address receivers 90 a andthe command receivers 90 b. The receiver 90 is activated in a periodwhen the enable signal REN is at a high level, and deactivated when theenable signal REN is at a low level. In the meantime, the chip selectreceiver 91 which receives the chip select signal CS is constantlyactivated.

As shown in FIG. 2, the internal chip select signal ICS1 and theinternal chip select signal ICS2 passed through the latency controlcircuit 100 are supplied to the selector 141. The selector 141 selectseither one of the internal chip select signals ICS1 and ICS2 based onthe mode signal CALEN, and supplies the selected signal to a circuitblock 80 as an internal chip select signal ICS3. Specifically, if themode signal CALEN is deactivated to a low level, i.e., set to the CALOFFmode, the selector 141 selects the internal chip select signal ICS1 themode signal CALEN is activated to a high level, i.e., set to the CALONmode, the selector 141 selects the internal chip select signal ICS2.

The circuit block 80 includes an address latch circuit 80 a and acommand decoder 80 b. If the internal chip select signal ICS3 isactivated, the circuit block 80 enables an internal address signal IADD1and an internal command signal ICMD1. The internal address signal IADD1refers to the output signals of the address receivers 90 a. The internalcommand signal ICMD1 refers to the output signals of the commandreceivers 90 b.

If the internal chip select signal ICS3 is activated, the address latchcircuit 80 a latches the internal address signal IADD1 output from theaddress receivers 90 a, and outputs the latched signal as an internaladdress signal IADD2. If the internal chip select signal ICS3 isactivated, the command decoder circuit 80 b decodes the internal commandsignal ICMD1 output from the command receivers 90 b, and outputs theresultant as an internal command signal ICMD2. The internal addresssignal IADD2 latched in the address latch circuit 80 a is supplied tothe row decoder 12, the column decoder 13, the mode register 25, and thelike according to the content of the internal command signal ICMD2.

Turning to FIG. 5, in this example, combinations of the chip selectsignal CS and the command signal CMD produce internal commands includinga DESEL command, a NOP command, an active command TACT, a prechargecommand IPRE, a write command IWR1, a read command IRD1, and a moderegister setting command MRS.

The DESEL command is a command that is generated when the chip selectsignal CS is in an inactive state. When the DESEL command is issued, theaccess control circuit 20 stops recognition of internal commandsgenerated by combinations of command signals CMD other than the DESELcommand. In other words, the generation of new internal commands otherthan the DESEL command is prevented. The access control circuit 20therefore issues no new command (new control) to the subsequent circuits(such as the row decoder 12). Consequently, the subsequent circuitsmaintain their state corresponding to the previous command. The NOPcommand is a command that is generated when the chip select signal CS isin an active state and all the bits (ACT, RAS, CAS, and WE of thecommand signal CMD are at a low level. Again, when the NOP command isissued, the access control circuit 20 issues no new command (newcontrol) to the subsequent circuits (such as the row decoder 12).Consequently, the subsequent circuits maintain their state correspondingto the previous command. As can be seen from FIG. 5, the DESEL commandis a broader command than the NOP command.

When the active command IACT, the write command IWR1, and the readcommand IRD1 are issued, the access control circuit 20 performs theforegoing operations to make a row access, a write access, and a readaccess, respectively. The precharge command IPRE is a command fordeactivating the memory cell array 11 which has been activated by theactive command IACT. The mode register setting signal MRS is an internalcommand for rewriting a set value of the mode register 25.

The configuration of the semiconductor device 10 a according to thefirst embodiment has been described so far. Next, the operation of thesemiconductor device 10 a according to the present embodiment will bedescribed.

Turning to FIG. 6, when the CALON mode is set, the issuance timing ofthe chip select signal CS from the controller 50 is not the same as thatof the command signal CMD and the address signal ADD. The command signalCMD and the address signal ADD are issued after a lapse of the firstlatency since the issuance of the chip select signal CS. FIG. 6 shows acase where the first latency is set to three clock cycles.

As shown in FIG. 6, when the chip select signal CS is issued at timet11, the internal chip select signal ICS1 changes to a high level andthe enable signal REN is activated to a high level. Consequently, theaddress receivers 90 a and the command receivers 90 b which have beendeactivated are activated to allow the reception of the address signalADD and the command signal CMD. Note that it takes some time to changethe input stages of the receivers 90 a and 90 b from an inactive stateto an active state. In FIG. 6, the gentle change of the enable signalREN represents the time needed. The number of input stages included inthe receivers 90 a and 90 b is approximately twenty, the same as thenumber of address terminals 21 and command terminals 22. To switch theinput stages from an OFF state to an ON state, all the gate electrodesincluded in the circuits need to be charged up. This produces relativelyhigh charging and discharging currents. In other words, a considerableamount of power is consumed to switch the receivers 90 a and 90 b froman inactive state to an active state and from an active state to aninactive state.

The internal chip select signal ICS1, is passed through the flip-flopcircuits FF1 to FF3 included in the latency control circuit 100 andoutput as the internal chip select signal ICS2 three clock cycles later.The activation timing of the internal chip select signal ICS2 is insynchronization with the timing when the command signal CMD and theaddress signal ADD are issued from the controller 50. As a result, thecommand signal CMD and the address signal ADD are processed by theaddress latch circuit 80 a and the command decoder 80 b. In FIG. 6, thecommand and address corresponding to time t11 are denoted by A.

Now, when the output signal CALm2 of the flip-flop circuit FF1 isactivated, the count value COUNT of the bit counter 120 is preset to themaximum value=7. The count value COUNT of the bit counter 120 is counteddown in synchronization with the internal clock signal ICLK. In theexample shown in FIG. 6, the chip select signal CS is activated again attime t12 before the count value COUNT of the bit counter 120 reaches 0.Time t12 is the timing where seven clock cycles have elapsed since timet11. Since the count value COUNT of the bit counter 120 is restored tothe maximum value=7 before reaching 0, the reset signal RST is notactivated at this point in time.

In the example shown in FIG. 6, the chip select signal CS is alsoactivated at time t13 which is two clock cycles after time t12.Operations based on the chip select signals CS input at times t12 andt13 are the same as those based on the chip select signal CS input attime M. Consequently, the enable signal REN remains at a high levelduring that time, allowing the reception of the address signal ADD andthe command signal CMD. In FIG. 6, the commands and addressescorresponding to times t12 and t13 are denoted by B and C respectively.

After a lapse of eight clock cycles since time t13, the count valueCOUNT of the bit counter 120 reaches 0, whereby the reset signal RST isactivated. In response to this, the SR, latch circuit L included in thereceiver control circuit 200 is reset, and the enable signal REN isdeactivated to a low level. At this point in time, it is assured thatthe chip select signal CS has not been activated for at least eightclock cycles. Then, no address signal ADD or command signal CMD will besupplied from the controller 50.

As described above, the semiconductor device 10 a according to thepresent embodiment does not deactivate the address receivers 90 a or thecommand receivers 90 b immediately after a lapse of the first latencysince the activation of the chip select signal CS. Instead, thesemiconductor device 10 a deactivates the address receivers 90 a and thecommand receivers 90 b after a lapse of the second latency longer thanthe first latency. The receivers 90 a and 90 b are therefore maintainedin an active state if the activation and deactivation of the chip selectsignal CS are frequently repeated in a short period. This can reducecharging and discharging currents resulting from the control of thereceivers 90 a and 90 b. Note that the address receivers 90 a and thecommand receivers 90 b are activated immediately after the activation ofthe chip select signal CS, without waiting for the first latency toelapse. The address signal ADD and the command signal CMD can thus beproperly received even if it takes time to activate the receivers 90 aand 90 b.

Next, a second embodiment of the present invention will be described.

Turning to FIG. 7, the second embodiment differs from the semiconductordevice 10 a shown in FIG. 2 in that there are added a latency shifter310 and an AND gate circuit or synchronization control circuit 320. Inother respects, the present embodiment is the same as the semiconductordevice 10 a shown in FIG. 2. The same components will thus be designatedby like reference numerals. Redundant description will be omitted.

The latency shifter 310 is a counter that counts the latency of a columnsystem command (such as a write command and a read command) included inthe internal command signal ICMD2 generated by the command decoder 80 b.Receiving a column-system internal command signal ICMD2, the latencyshifter 310 counts up a predetermined latency and then outputs thecolumn-system internal command signal ICMD2 as an internal commandsignal ICMD3. Latencies for the latency shifter 310 to count include thelatency of a write command, or a write latency WL, and the latency of aread command, or a read latency RL.

The write latency WL refers to the latency from the issuance of a writecommand from the controller 50 to the input of the first piece of writedata DQ. Issuance timing of a write command may be preceded by anadditive latency (AL) than its original issuance timing. The writelatency WL is thus defined by WL=AL+CWL, where a latency from theoriginal issuance timing of a write command to the input of the firstpiece of write data DQ is the CAS write latency (CWL).

The read latency RL refers to the latency from the issuance of a readcommand from the controller 50 to the output of the first piece of readdata DQ. Issuance timing of a read command may be preceded by anadditive latency (AL) than its original issuance timing. The readlatency RL is thus defined by RL=AL+CL, where a latency from theoriginal issuance timing of a read command to the output of the firstpiece of read data DQ is the CAS latency (CL). The controller 50 issuesthe write latency WL, the read latency RL, and the additive latency ALto the semiconductor device 10 b in advance. The semiconductor device 10b stores the values of the write latency WL the read latency RL, and theadditive latency AL into the mode register 25 shown in FIG. 2.

The latency shifter 310 operates in synchronization with an internalclock signal ICLK2 output from the synchronization control circuit 320.The synchronization control circuit 320 is a two-input AND gate circuitthat receives the internal clock signal ICLK and the enable signal REN.The synchronization control circuit 320 is thus clocked only in a periodwhere the enable signal REN is at a high level. In the presentembodiment, the second latency is set to be longer than the writelatency WL and the read latency RL.

Turning to FIG. 8, in the present example, the count value COUNT of thebit counter 120 is preset to a maximum value of 11.

As shown in FIG. 8, when the chip select signal CS is issued at timet21, the internal chip select signal ICS1 changes to a high level andthe enable signal REN is activated to a high level. Consequently, theaddress receivers 90 a and the command receivers 90 b which have beendeactivated are activated to allow the reception of the address signalADD and the command signal CMD. Such an operation is the same as in thefirst embodiment.

When the enable signal REN is activated to a high level, the internalclock signal ICLK2 starts clocking. The clocking of the internal clocksignal ICLK2 makes the latency shifter 310 operable. After a lapse ofthree clock cycles since the activation of the chip select signal CS,the controller 50 issues the command signal CMD and the address signalADD. The present example deals with the case where the command signalCMD issued from the controller 50 is a write command. Such a commandsignal CMD is accepted by the command receivers 90 b, and decoded intothe internal command signal ICMD2 by the command decoder 80 b. Theinternal command signal ICMD2 is input to the latency shifter 310.

At this point in time, the latency shifter 310 is in an operable state.The latency shifter 310 therefore provides a delay as much as the writelatency WL to the internal command signal ICMD2 input to the latencyshifter 310, and outputs the resultant as the internal command signalICMD3. In the present embodiment, the latency that represents a writelatency WL and a read latency RL for the latency shifter 310 to providemay be referred to as a “third latency”.

After a lapse of twelve clock cycles since time t21, the count valueCOUNT of the bit counter 120 reaches 0, whereby the enable signal REN isdeactivated to a low level (time t22). In response to this, the internalclock signal ICLK2 stops clocking. At this point in time, the chipselect signal CS has not been activated for at least twelve clockcycles. This ensures that no command is retained in the latency shifter310. The reason is that the second latency is set to be longer than thewrite latency WL and the read latency RL.

Incidentally, the entire contents disclosed in the aforementioned patentdocuments and non-patent documents including Japanese Patent ApplicationLaid-Open No. 2000-285674 are incorporated herein by reference.

As described above, the semiconductor device 10 b according to thesecond embodiment stops clocking the internal clock signal ICLK2 in aperiod where the enable signal REN is in an inactive state.Consequently, useless clocking is avoided in a period where no commandis retained in the latency shifter 310. This allows a further reductionof power consumption. In addition to the effects of the firstembodiment.

It is apparent that the present invention is not limited to the aboveembodiments, but may be modified and changed without departing from thescope and spirit of the invention.

Volatile memories, non-volatile memories, or mixtures of them can beapplied to the memory cells of the present invention.

The technical concept of the present invention may be applied to asemiconductor device including a signal transmission circuit. The formsof the circuits in the circuit blocks disclosed in the drawings andother circuits for generating the control signals are not limited to thecircuit forms disclosed in the embodiments.

When the transistors are field effect transistors (FETs), various FETsare applicable, including MIS (Metal Insulator Semiconductor) and TFT(Thin Film Transistor) as well as MOS (Metal Oxide Semiconductor). Thedevice may even include bipolar transistors. For example, the presentinvention can be applied to a general semiconductor device such as a CPU(Central Processing Unit), an MCU (Micro Control Unit), a DSP (DigitalSignal Processor), an ASIC (Application Specific Integrated Circuit),and an ASSP (Application Specific Standard Circuit), each of whichincludes a memory function. An SOC (System on Chip), an MCP (Multi ChipPackage), and a POP (Package on Package) and so on are pointed to asexamples of types of semiconductor device to which the present inventionis applied. The present invention can be applied to the semiconductordevice that has these arbitrary product form and package form.

When the transistors that constitute a logic gate and the like are fieldeffect transistors (FETs), various FETs are applicable, including MIS(Metal insulator Semiconductor) and TFT (Thin Film Transistor) as wellas MOS (Metal Oxide Semiconductor). The device may even include bipolartransistors.

In addition, an NMOS transistor (N-channel MOS transistor) is arepresentative example of a first conductive transistor, and a PMOStransistor (P-channel MOS transistor s a representative example of asecond conductive transistor.

Many combinations and selections of various constituent elementsdisclosed in this specification can be made within the scope of theappended claims of the present invention. That is, it is needles tomention that the present invention embraces the entire disclosure ofthis specification including the claims, as well as various changes andmodifications which can be made by those skilled in the art based on thetechnical concept of the invention.

In addition, while not specifically claimed in the claim section, theapplicant reserves the right to include in the claim section of theapplication at any appropriate time the following information processingsystems:

A1. An information processing system comprising:

a controller that outputs a chip select signal and a command signal; and

a semiconductor device including:

-   -   a command receiver receiving the command signal supplied from        the controller to generate an internal command signal; and    -   a control circuit activating an internal chip select signal        after elapse of a first latency since the chip select signal        supplied from the controller is activated,

wherein the control circuit activates the command receiver in responseto the chip select signal, and deactivates the command receiver when thechip select signal is maintained in an inactive state during a secondlatency that is larger than the first latency.

A2. The information processing system as described in A1, wherein thesemiconductor device further includes:

a command decoder that decodes the internal command signal to generate afirst internal command signal; and

a selector that supplies either one of the chip select signal and theinternal chip select signal to the command decoder to activate thecommand decoder.

A3. The information processing system as described in A1 or A2, wherein

the semiconductor device further includes an address receiver receivingan address signal supplied from the controller to generate an internaladdress signal,

the control circuit activates the address receiver in response to thechip select signal, and deactivates the address receiver when the chipselect signal is maintained in the inactive state during the secondlatency.

What is claimed is:
 1. A method for controlling a memory devicecomprising: supplying a clock signal from a controller to the memorydevice; supplying a chip select signal from the controller to the memorydevice; supplying a mode register setting command from the controller tothe memory device to set an internal mode signal in the memory device;supplying an access command from the controller to the memory device;wherein the controller activates the chip select signal at the same timeas the access command if the mode register setting command sets theinternal mode signal to a deactivated level and the controller activatesthe chip select signal a first predetermined number of cycles of theclock signal earlier than the access command if the mode registersetting command sets the internal mode signal to an activated level; andwherein the memory device delays the chip select signal by the firstpredetermined number of cycles of the clock signal to provide a firstinternal chip select signal, selects the chip select signal as a secondinternal chip select signal when the internal mode signal isdeactivated, selects the first internal chip select signal as the secondinternal chip select signal when the internal mode signal is activated,and decodes the access command to provide first internal command signalswhen the second internal chip select signal is activated.
 2. The methodas claimed in claim 1, further comprising supplying an address signalfrom the controller to the memory device at the same time as the accesscommand, wherein the memory device latches the address signal when thesecond internal chip select signal is activated.
 3. The method asclaimed in claim 1, wherein the memory device delays the chip selectsignal by the first predetermined number of cycles of the clock signalto provide a first internal chip select signal when the internal modesignal is deactivated.
 4. The method as claimed in claim 1, wherein thefirst predetermined number of cycles of the clock signal is a variableset by the controller with the mode register setting command.
 5. Themethod as claimed in claim 1, wherein the first predetermined number ofcycles of the clock signal is a fixed number.
 6. The method as claimedin claim 5, wherein the fixed number is
 3. 7. The method as claimed inclaim 1, wherein the memory device activates circuits receiving theaccess command in response to the chip select signal when the internalmode signal is activated.
 8. The method as claimed in claim 7, whereinthe memory device deactivates the circuits receiving the access commanda second predetermined number of cycles of the clock signal after thechip select signal is deactivated.
 9. The method as claimed in claim 8,wherein the second predetermined number of cycles is greater than thefirst predetermined number of cycles.
 10. The method as claimed in claim1, wherein the access command is a selected one of an activationcommand, a precharge command, a write command, and a read command. 11.The method as claimed in claim 1, wherein the memory device delays thefirst internal command signals by a second predetermined number ofcycles of the clock signal to provide a second internal command signals.12. The method as claimed in claim 11, wherein the second predeterminednumber of cycles of the clock signal is an additive latency when theaccess command is a selected one of a write command and a read command.